Hard-edged polygons with normal maps
Sorry for any trouble, but I was able to find the answer in one of the Blender docs. The real culprit was the Image Texture I was using for body_normals.png:


  (emphasisis added by me) 
  Generate a perturbed normal from an RGB normal map image. This is usually chained with an Image Texture node in the color input, to specify the normal map image. For tangent space normal maps, the UV coordinates for the image must match, and the image texture should be set to Non-Color mode to give correct results.


I guess I must have skipped over this part in this project. Switching to Non-Color data in the Image Texture fixed my problem :). Now there is are no hard-edged polygon shading weirdness anymore.